This application is based on the application No. 2002-035011 filed in Japan on Feb. 13, 2002, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a projection optical system, in which a light beam emitted from a light source is decomposed into light beams of three primary colors (i.e., red, blue and green), which illuminate three reflection type liquid crystal light valves (hereinafter abbreviated as xe2x80x9cLCLVsxe2x80x9d), and then, images generated respectively by the LCLVs are synthesized into a single image to be thus projected. Furthermore, the present invention relates to a projector using the above-described projection optical system.
2. Description of the Related Art
There has been known an optical system disclosed in, for example, Japanese Patent Application Laid-open No. 271,683/1999 (hereinafter abbreviated as xe2x80x9cJP ""683xe2x80x9d) as a projection optical system for synthesizing images generated by three LCLVs.
FIG. 14, which corresponds generally to FIG. 1 of JP ""683, discloses an optical system including a color-separating section, an image-generating section, and an image-synthesizing section. The color-separating section includes: a first color-separating mirror 401 for separating a white polarized optical beam Ws into a blue light beam Bs and a yellow light beam including a green light beam Gs and a red light beam Rs; a half-wave plate 406 for rotating the polarization direction of the blue light beam Bs 90 degrees; a second color-separating mirror 402 for synthesizing the green light beam Gs and the blue light beam Bp while separating the red light beam Rs. The image-generating section includes: a first LCLV 411 for modulating the red color light beam Rs thus generating a red image light beam; a second LCLV 412 for modulating the green color light beam Gs thus generating a green image light beam; a third LCLV 414 for modulating the blue color light beam Bp thus generating a blue image light beam. The image-synthesizing section includes: a first polarizing beam splitter (hereinafter abbreviated as xe2x80x9cPBSxe2x80x9d) 409 for separating the green light beam Gs and the blue light beam Bp, so as to guide the separated light beams to the second LCLV 412 and the third LCLV 413, respectively, and further, for synthesizing each of the image light beams reflected on the LCLVs 412 and 413, respectively, with each other; a second PBS 410 for guiding the red light beam to the first LCLV 411 and emitting the first image light beam Rp reflected on the first LCLV 411; a second phase plate 407, disposed external to an output surface of the first PBS 409, for rotating the red image light beam Rp 90 degree; a dichroic prism 414 for synthesizing the green and blue image light beams, Gp and Bp, emitted from the first PBS 409 and the red light beam Rp emitted from the second phase plate 407 with each other; three band-pass filters 511, 512, and 513 for eliminating an unwanted another color light beam, respectively. Here, the image light beam is a light component having image information out of light beams modulated by the LCLV. Additionally, subscript p means P polarized light, and subscript s means S polarized light.
However, the contrast of an image still has plenty of room for improvement in the optical system disclosed in JP ""683. The image-synthesizing section in the projection optical system is configured such that the blue image light beam is reflected on the PBS 409, to be thus emitted toward the dichroic prism 414. In view of the characteristics of the PBS, S polarized light beam is perfectly reflected on PBS, and so, a transmitted P polarized light beam has a great polarization degree. But, as transmittance of a P polarized light beam is not 100 percent, P polarized light beam which should be inherently transmitted is reflected by several percentages, and then, the reflected P polarized light beam is mixed into a reflected S polarized light beam. The blue image light beam contains an unnecessary P polarized light beam, and so, the contrast of the blue image is degraded.
The present invention has been accomplished in an attempt to solve the above problems observed in the prior art. An object of the present invention is to provide a projection optical system that high contrast image can be obtained. Furthermore, another object of the present invention is to provide a projector having the above-described projection optical system.
In order to achieve the above-described objects, a first projection optical system according to the present invention includes: a first color-separating mirror for separating a substantially polarized light into a first color light and a color light including a second color light and a third color light; a first phase plate for rotating a plane of polarization of the color light or the third color light; a second color-separating mirror for synthesizing the first color light and the third color light with each other and separating the second color light; a first LCLV for modulating the first color light thus generating a first image light; a second LCLV for modulating the second color light thus generating a second image light; a third LCLV for modulating the third color light thus generating a third image light; a first PBS for separating the first color light and the third color light from each other, so as to guide the separated color lights to the first LCLV and the third LCLV, respectively, and for synthesizing the first image light and the third image light; a second PBS for guiding the second color light to the second LCLV and emitting the second image light; a second phase plate disposed external to an output surface of the first PBS, the first and third image lights being emitted from the output surface; and a third PBS for synthesizing the first and third image lights emitted from the second phase plate and the second image light emitted from the second PBS.
In order to achieve the above-described objects, a second projection optical system according to the present invention includes: a first LCLV for modulating a first color light thus generating a first image light; a second LCLV for modulating a second color light thus generating a second image light; a third LCLV for modulating a third color light thus generating a third image light; a first PBS for separating the first color light and the third color light different in color and in plane of polarization each other, so as to guide the separated lights to the first LCLV and the third LCLV, respectively, and further, for synthesizing the first image light and the third image light; a second PBS for guiding a polarized second color light to the second LCLV and emitting the second image light; a half-wave plate disposed external to an output surface of the first PBS, the first and third image lights being emitted from the output surface; and a third PBS for synthesizing the first and third image lights emitted from the half-wave plate and the second image light emitted from the second PBS, wherein the third PBS reflects the second image light thereby while passes the first image light therethrough by a dichroic characteristic of the third PBS, and wherein, with respect to the third color light, the third PBS reflects a S polarized light thereby while passes a P polarized light therethrough.
In addition, in order to solve the above-described problems, a projector according to the present invention includes the first projection optical system or the second projection optical system.